There are three dialects of spoken Irish: Ulster (now predominantly in County Donegal), Connacht (Counties Mayo and Galway), and Munster (Counties Kerry, Cork, and Waterford). Some spelling conventions are common to all the dialects, while others vary from dialect to dialect. In addition, individual words may have in any given dialect a pronunciation that is not reflected by the spelling (the pronunciation in this article reflects Connacht Irish pronunciation; other accents may differ, but are occasionally included).

The acute accent over the vowels is ignored for purposes of alphabetization. Modern loanwords also make use of j k q v w x y z. Of these, v is the most common. It occurs in a small number of words of native origin in the language such as vácarnach, vác and vrác, all of which are onomatopoeic. It also occurs in a number of alternative colloquial forms such as víog instead of bíog and vís instead of bís as cited in Niall Ó Dónaill's Foclóir Gaeilge–Béarla (Irish–English Dictionary). It is also the only non-traditional letter used to write foreign names and words adapted to the Irish language (for example, Switzerland, or Helvetia, is Gaelicised as An Eilvéis; Azerbaijan, in contrast, is written An Asarbaiseáin rather than *An Azarbaijáin). The letters j, q, w, x, y and z are used primarily in scientific terminology or direct, unaltered borrowings from English and other languages, although the phoneme /z/ does exist naturally in at least one dialect, that of West Muskerry, County Cork, as the eclipsis of s. k is the only letter not to be listed by Ó Dónaill. h, when not prefixed to an initial vowel as an aspirate in certain grammatical functions (or when not used as an indicator of lenition when Roman type is used), occurs primarily in loanwords as an initial consonant. The letters' names are spelt out thus:

á bé cé dé é eif gé héis í eil eim ein ó pé ear eas té ú

along with jé cá cú vé wae eacs yé zae.

Tree names were once popularly used to name the letters. Tradition taught that they all derived from the names of Ogham letters, though it is now known that only some of the earliest Ogham letters were named after trees.

Prior to the middle of the 20th century, Irish was usually written using Gaelic script. This typeface, together with Roman type equivalents and letter name pronunciations along with the additional lenited letters, is shown below.

Use of Gaelic type is today almost entirely restricted to decorative and/or self-consciously traditional contexts. The dot above the lenited letter is usually replaced by a following h in the standard Roman alphabet [for example, ċ in Gaelic type becomes ch in Roman type]. The only other use of h in Irish is for vowel-initial words after certain proclitics (e.g. go hÉirinn, "to Ireland") and for words of foreign derivation such as hata "hat".

Although the Gaelic script remained common until the mid-20th century, efforts to introduce Roman characters began much earlier. Theobald Stapleton's 1639 catechism was printed in a Roman type alphabet, and also introduced simplified spellings such as suí for suidhe and uafás for uathbhás, though these did not become standard for another 300 years.

The consonant letters generally correspond to the consonant phonemes as shown in this table. See Irish phonology for an explanation of the symbols used and Irish initial mutations for an explanation of eclipsis. In most cases, consonants are "broad" (velarised) when the nearest vowel letter is one of a, o, u and "slender" (palatalised) when the nearest vowel letter is one of e, i.

Sequences of vowels are common in Irish spelling due to the "caol le caol agus leathan le leathan" ("slender with slender and broad with broad") rule. This rule states that the vowels on either side of any consonant must be both slender (e or i) or both broad (a, o or u), to unambiguously determine the consonant's own broad vs. slender pronunciation.
An apparent exception is the combination ae, which is followed by a broad consonant despite the e.

In spite of the complex chart below, pronunciation of vowels in Irish is mostly predictable from a few simple rules.

Fada vowels (á, é, í, ó, ú) are always pronounced.

Vowels on either side of a fada (except for other fada vowels) most often do not spell any phoneme (there are several exceptions). Their presence is almost always necessary to simply satisfy the "caol le caol agus leathan le leathan" rule. These letters are not entirely silent, however. The fada vowel and the adjacent consonant require the tongue body to be in different positions, and these letters capture the transient sound produced while it is moving from one position to the other.

Between a consonant and a broad vowel, e and i are usually non-phonemic in the same way. This applies to:

The short vowels io, oi and ui have multiple pronunciations that depend on adjacent consonants.

The following series of charts indicates how written vowels are generally pronounced. Each dialect has certain divergences from this general scheme, and may also pronounce some words in a way that does not agree with standard orthography.

Vowels with an acute accent (known in Irish as a fada or síneadh fada) are always pronounced long. In digraphs and trigraphs containing a vowel with an acute accent, only the vowel with the accent mark is usually pronounced, but there are several exceptions.

A vowel or digraph followed by i is usually pronounced as that vowel. The i is not pronounced in that case, and just indicates that the following consonants are slender. However, it may be pronounced in the digraphs ei, oi, ui.

Irish spelling makes use today of only one diacritic, and formerly used a second. The acute accent (Irish: síneadh fada "long sign") is used to indicate a long vowel, as in bád/bˠaːd̪ˠ/ "boat". However, there are some circumstances under which a long vowel is not indicated by an acute accent, namely:

The overdot (Irish: ponc séimhithe "dot of lenition", buailte "struck", or simply séimhiú, "lenition") was formerly used, especially in Gaelic script, to indicate the lenited version of a consonant; currently a following letter h is used for this purpose. Thus the letters ḃ ċ ḋ ḟ ġ ṁ ṗ ṡ ṫ are equivalent to bh ch dh fh gh mh ph sh th. In Old Irish orthography, the dot was used only for ḟ ṡ, while the following h was used for ch ph th; lenition of other letters was not indicated. Later the two systems spread to the entire set of lenitable consonants and competed with each other. Eventually the standard practice was to use the dot when writing in Gaelic script and the following h when writing in Roman letters.

As with most European languages such as French, Spanish or German, Irish diacritics must be preserved in uppercase forms. If diacritics are unavailable (for example, on a computer using ASCII), there is no generally accepted standard for replacing it (unlike some languages like German, where the umlaut is replaced by a following e and ß is replaced by ss), and so it is generally just omitted entirely.

Lower-case i has no tittle in Gaelic script, and road signs in the Republic of Ireland, which use a typeface based on Transport, also use a dotless lowercase i (as well as a Latin alpha glyph for a). However, as printed and electronic material like books, newspapers and web pages use Roman hand almost invariably, the tittle is generally shown but it is not a diacritic and has no significance. (In Irish, the graphemic distinction between dotted i and dotless ı does not arise, i.e. they are not different letters as they are in, for example, Turkish and Azeri).

According to Alexei Kondratiev,[citation needed] the dotless i was developed by monks in the manuscripts to denote the modification of the letter following it. In the word go deimhin for example, the first i would be dotless, softening the m, and the second dotted i would be a normal vowel. The dotting of every occurrence of i in Irish became a convention, as did the letter h, when the language became more usually typed than handwritten, and the limitations of the machine to accommodate a scribe's flicks and notations imposed standardization. This meant that "letters" that were more intended to modify other letters (h and dotless i) became equal letters. In this process formally notation letters became emboldened and distracting to non-initiates. Moves in signage to replace instances of h with dots, and possibly also replace dotless i with an under-dot, for example, would clarify spelling and make words less cluttered with notation letters and easier to read. Removing notation letters (h and dotless i) would also constitute a spelling reform without having to change the essential spellings. The dots or diacritics would take the place of distracting notational letters as was once common in manuscripts and handwriting prior to keyboards.

In general, punctuation marks are used in Irish much as they are in English. One punctuation mark worth noting is the Tironian et ⁊ which is generally used to abbreviate the word agus "and", much as the ampersand is generally used to abbreviate the word and in English.

The hyphen (Irish: fleiscín) is used in Irish after the letters t and n when these are attached to a vowel-initial word through the rules of the initial mutations, as in an t-arán "the bread", a n-iníon "their daughter". However, the hyphen is not used when the vowel is capitalised, as in an tAlbanach "the Scotsman", Ár nAthair "Our Father". No hyphen is used with the h that is attached to a vowel-initial word: a hiníon "her daughter".

The hyphen is also used in compound words under certain circumstances:

The apostrophe (Irish: uaschama) is used to indicate an omitted vowel in the following cases:

the prepositions de "from" and do "to" both become d’ before a vowel (or fh + vowel, since fh is silent), as in Thit sí d'each "She fell from a horse" and Tabhair d'fhear an tí é "Give it to the landlord"

the possessive pronouns mo "my" and do "your (singular)" become m’ and d’ before a vowel or fh + vowel, as in m'óige "my youth", d'fhiacail "your tooth"

the preverbal particledo becomes d’ before a vowel or fh + vowel, as in d'ardaigh mé "I raised", d'fhanfadh sé "he would wait"

the copular particle ba becomes b’ before a vowel or fh + vowel, as in B'ait liom é sin "I found that odd" and b'fhéidir "maybe". However, ba retains its vowel before the pronouns é, í, iad, as in Ba iad na ginearáil a choinnigh an chumhacht "It was the generals who kept the power"

Capitalisation rules are similar to English. However, a prefix letter remains in lowercase when the base initial is capitalised (an tSín "China"). For text written in all caps, the prefix letter is often kept in lowercase, or small caps (STAIR NA HÉIREANN "THE HISTORY OF IRELAND").[1]
An initial capital is used for:[2]

Irish has a number of abbreviations, most of which, like lch. for leathanach ("p."/"page") and m.sh. for mar shampla ("e.g."/"for example" "exempli gratia") are straightforward. Two that may require explanation are .i. (which begins and ends with a full stop) for eadhon ("i.e."/"that is") and ⁊rl. or srl. for agus araile ("etc."/"and so forth" "et cetera").

The booklet was expanded in 1947, and republished as An Caighdeán Oifigiúil ("the official standard") in 1958, combined with the standard grammar of 1953.[9] It attracted initial criticism as unhistorical and artificial; some spellings fail to represent the pronunciation of some dialects, while others preserve letters not pronounced in any dialect.[9] Its status was reinforced by use in the civil service and as a guide for Tomás de Bhaldraithe's 1959 English–Irish dictionary and Niall Ó Dónaill's 1977 Irish–English dictionary.[9] A review of the written standard, including spelling, was announced in 2010, with a view to improving "simplicity, internal consistency, and logic".[10] The result was the 2017 updated Caighdeán Oifigiúil.[11]

^Central Translation Unit. "The Scope of the Process". Review of Caighdeán Oifigiúil na Gaeilge. Department of Community, Equality and Gaeltacht Affairs. Archived from the original on 5 October 2013. Retrieved 12 February 2012.

^"Rannóg an Aistriúcháin > An Caighdeán Oifigiúil". In September 2014, members of the public and other interested parties were asked to make submissions regarding An Caighdeán Oifigiúil. An Advisory Committee was also established, which worked tirelessly for a year and a half to identify issues and to make recommendations. The result of this work is the new edition of An Caighdeán Oifigiúil, published by the Houses of the Oireachtas Service in 2017.